Method and apparatus for inspecting sample surface

ABSTRACT

Provided is a method and an apparatus for inspecting a sample surface with high accuracy. Provided is a method for inspecting a sample surface by using an electron beam method sample surface inspection apparatus, in which an electron beam generated by an electron gun of the electron beam method sample surface inspection apparatus is irradiated onto the sample surface, and secondary electrons emanating from the sample surface are formed into an image toward an electron detection plane of a detector for inspecting the sample surface, the method characterized in that a condition for forming the secondary electrons into an image on a detection plane of the detector is controlled such that a potential in the sample surface varies in dependence on an amount of the electron beam irradiated onto the sample surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.13/398,112, filed Feb. 16, 2012, which is a Divisional of U.S.application Ser. No. 12/162,067, filed Jul. 24, 2008, which is anational stage application filed under 35 USC §371 of InternationalApplication No. PCT/JP2007/051047, filed Jan. 24, 2007, which claims thebenefit of priority from Japanese Application No. 2006-16519, filed onJan. 25, 2006, Japanese Application No. 2006-58847, filed on Mar. 6,2006 and Japanese Application No. 2006-58862, filed on Mar. 6, 2006, theentire disclosures of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to an inspection apparatus and aninspection method for inspecting a pattern formed in a surface of asample and in particular, to a projection type electron beam sampleinspection apparatus for inspecting or evaluating a pattern (forexample, an overlay mark pattern) formed in a sample, such as a wafer ora substrate, by irradiating an electron beam onto the surface of thesample and an inspection method for inspecting or evaluating the patternby using the same inspection apparatus.

BACKGROUND ART

A semiconductor manufacturing process involves the steps of exposing,etching and thin-film deposition, which steps are repeated several or adozen times. One critical factor in those steps is defects that could becreated in respective steps, and so the detection of electrical defectscan be critical among others. In addition, matching (overlay) oflocations between a wiring pattern formed in an under layer and a wiringpattern to be formed in an upper layer in a plurality of wiring patternsstacked one on top of the other is also critical.

It is extremely difficult for a conventional optical microscope todetect the electrical defect and it takes a long inspection period for aSEM (Scanning type Electron Microscope) to make an inspection over alarge area.

Further, in such an inspection apparatus using an electron beam, aneffect from charge-up in a sample surface could inhibit a clear imagefrom being obtained. Further, in a conventional approach, the matchinghas been provided, for example, by making an alignment (an overlayinspection) by means of a light (an optical microscope) in conjunctionwith a mark of specified purpose (an overlay mark) employed foralignment of the locations between the pattern in the under layer andthe pattern in the upper layer.

REFERENCE

-   Patent document: U.S. Pat. No. 6,091,249

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

As described above, in the inspection of the sample surface by using theelectron beam, a resultant image could lack focus due to the charge-upin the sample surface caused by the electron beam irradiation. Inaddition, since the overly mark has a different pattern size from thatof the actual device pattern, such an overlay inspection using light issubject to an effect of coma aberration from the light, often resultingin an alignment offset in which the actual device pattern is shifted andexposed to the light, even if the overlay mark is within a tolerancerange in the alignment. Further, since the overly mark has a differentpattern size from that of the actual device pattern, such overlayinspection by means of the light as described above is subject to aneffect of coma aberration from the light, often resulting in thealignment offset in which the actual device pattern is shifted andexposed to the light, even if the overlay mark is within a tolerancerange in the alignment.

The present invention has been made in the light of the problems pointedabove, and an object thereof is to provide an inspection method and aninspection apparatus for inspecting a sample surface with highlyimproved accuracy enabled by controlling a sample voltage with charge-upin the sample taken into account, which would not be provided by theprior art, in the sample surface inspection apparatus used in amanufacturing process of a semiconductor device.

Another object of the present invention is to provide an inspectionmethod and an inspection apparatus which enable a surface inspection tobe carried with high accuracy by modifying a sample voltage independence on an amount of the electron beam.

Still another object of the present invention is to provide aninspection apparatus and an inspection method for inspecting a samplewith high efficiency and high accuracy enabled by controlling a stagesuch that the stage may be moved in synchronism with an operatingfrequency of a sensor during inspection and a time required to move thestage may be minimized during the stage being moved between patterns tobe inspected.

Another object of the present invention is to provide a projection typeinspection apparatus or an inspection method for inspecting a surfaceadapted to improve a speed and accuracy in an inspection of a pattern bycontrolling an irradiation geometry of an electron beam relative to apattern so as to be associated with a movement of a sample or byelongating it along the moving direction of the sample.

Means to Solve the Problem

According to a first aspect of the present invention, provided is amethod for inspecting a sample surface by using an electron beam methodsample surface inspection apparatus, in which an electron beam generatedby an electron gun of the electron beam method sample surface inspectionapparatus is irradiated onto the sample surface, and secondary electronsemanating from the sample surface are formed into an image toward anelectron detection plane of a detector for inspecting the samplesurface, the method characterized in that a condition for forming thesecondary electrons into an image on the detection plane of the detectoris controlled such that a potential in the sample surface varies independence on an amount of the electron beam irradiated onto the samplesurface.

In one method for controlling the image forming condition for thesecondary electrons in the present invention, a sample voltage or aretarding voltage may be modified in dependence on an amount of theelectron beam irradiated onto the sample surface. Further, the detectormay be an EB-CCD. It is to be noted that the detector may comprise a MCPand a TDI-CCD.

According to another aspect of the present invention, provided is anelectron beam method sample surface inspection apparatus, comprising: anelectron gun for generating an electron beam to be irradiated onto asample surface; a primary optical system for guiding the electron beamonto the sample surface; a detector for detecting secondary electronsemanating from the sample surface; and a secondary optical system forguiding the secondary electrons onto the detector, the apparatuscharacterized in further comprising a voltage adjustment mechanism formodifying a potential in the sample surface in dependence on an amountof the electron beam.

In the invention as designated above, the apparatus may comprise as thevoltage adjustment mechanism a means for modifying the sample voltage orthe retarding voltage in dependence on an amount of the electron beamirradiated onto the sample surface.

Further, the detector may be an EB-CCD. It is to be noted that thedetector may comprise a MCP and a TDI-CCD.

In the method and apparatus for inspecting a surface of a sample toprovide a defect inspection or the like for a semiconductor deviceaccording to the present invention as designated above, preferably, thesecondary optical system for guiding secondary electrons emanating fromthe sample surface in response to the electron beam irradiated onto thesemiconductor wafer to the detector may include a quadruple lens andfurther the method may include a step for forming the secondaryelectrons into an image by using a plurality of electrostatic lenses.

Further, the detector for the secondary electrons may include, inaddition to the MCP and the TDI-CCD as designated above, a fluorescentscreen between the MCP and the TDI-CCD. Alternatively, an EB-TDI may beused instead of the MCP and the TDI-CCD, or an EB-CCD may also be used.

In the present invention as designated above, the voltage adjustmentmechanism or the means for modifying the retarding voltage may comprisea stabilizing direct current power source for modifying an outputvoltage in accordance with an external signal and a computer serving forcontrolling the voltage modification, in which a command is input to thecomputer so that a resultant output value (output voltage) of thestabilizing current power source represents a desired value formodifying the potential in the sample surface.

According to another aspect of the invention, provided is an inspectionmethod for inspecting a pattern formed in a sample by using an electronbeam, the method characterized in that a stage holding a sample thereonis moved at a frequency in synchronism with an operating frequency of asensor during inspection of a pattern to be inspected and a moving speedof the stage is controlled so that a time required for movement isminimized during the stage being moved to another pattern to beinspected.

In the inspection method as designated above, the pattern to beinspected may include two or more patterns consisting of differentsectional structures or different materials, and a plurality of patternsmay be inspected concurrently.

According to another aspect of the invention, provided is an inspectionapparatus for inspecting a pattern formed in a sample by using anelectron beam, comprising: a holding mechanism for holding the sample; astage with the holding mechanism mounted thereon, and adapted to bemovable in at least one direction; an electron beam source forgenerating electrons for irradiation of the electron beam directed tothe sample; a first electro-optical system for guiding the electron beamgenerated from the electron beam source onto the sample for irradiationof the electron beam to the sample; a detector for detecting electronsemanating from the sample; and a second electro-optical system forguiding the electrons to the detector, the apparatus further comprisinga control unit to provide control so that the stage is moved at a speedin synchronism with an operating speed of the detector during inspectionof the pattern, and the stage is accelerated when being moved to anotherpattern on the sample.

In the inspection apparatus as designated above, the pattern to beinspected may include two or more patterns consisting of differentsectional structures or different materials, and a plurality of patternsmay be inspected concurrently.

According to another aspect of the invention, provided is an inspectionapparatus for inspecting a pattern formed on a sample by using anelectron beam, comprising: a holding mechanism for holding the sample; astage with the holding mechanism mounted thereon, and adapted to bemovable in at least one direction; an electron beam source forgenerating electrons for irradiation of the electron beam directed tothe sample; a first electro-optical system for guiding the electron beamgenerated from the electron beam source onto the sample for irradiationof the electron beam to the sample; a detector for detecting electronsemanating from the sample; and a second electro-optical system forguiding the electrons to the detector, wherein an irradiation geometryof the electron beam to the pattern defines an elongated irradiationgeometry that is longer than a length of the pattern along the movingdirection of the stage during the stage being moved serially forinspecting the pattern so that the pattern could have been previouslysubject to the irradiation of electrons.

Further, according to another aspect of the invention, provided is aninspection apparatus for inspecting a pattern formed on a sample byusing an electron beam, comprising: a holding mechanism for holding thesample; a stage with the holding mechanism mounted thereon, and adaptedto be movable in at least one direction; an electron beam source forgenerating electrons for irradiation of the electron beam directed tothe sample; a first electro-optical system for guiding the electron beamgenerated from the electron beam source onto the sample for irradiationof the electron beam to the sample; a detector for detecting electronsemanating from the sample; and a second electro-optical system forguiding the electrons to the detector, the inspection apparatus furthercomprising a control unit for controlling an irradiation geometry of theelectron beam in association with an operation of the stage so that theirradiation geometry of the electron beam onto the pattern allows thepattern to be previously subject to the irradiation of electrons duringthe stage of moving serially for inspecting the pattern.

In the sample surface inspection apparatus according to the presentinvention as designated above, the secondary optical system for guidingsecondary electrons emanating from the sample surface in response to theelectron beam irradiated onto the sample, such as a semiconductor wafer,to the detector may include a quadruple lens and the apparatus mayfurther include a step for forming the secondary electrons into an imageby using a plurality of electrostatic lenses.

Further, the detector for the secondary electrons emanating from thesample may comprise the TDI and the fluorescent screen.

Further, the detector for the secondary electrons may include, inaddition to the fluorescent screen and the TDI-CCD as designated above,a MCP located before the fluorescent screen.

Alternatively, an EB-TDI may be used instead of the MCP and the TDI-CCD,or an EB-CCD may also be used. Further, a combination of the MCP and theEB-CCD may be employed.

In the inspection apparatus as designated above, the pattern to beinspected may include two or more patterns consisting of differentsectional structures or different materials, and a plurality of patternsmay be inspected concurrently.

Effect of the Invention

As stated above, according to the present invention, in the inspectionof the surface of the sample, such as a wafer, a substrate and the like,in the manufacturing process of the semiconductor device, the surfaceinspection with high accuracy can be achieved by modifying the samplevoltage in dependence on an amount of the electron beam and thus qualityand throughput of the semiconductor device can be improved.

Further, in the sample surface inspection, the detection of the overlaycan be accomplished and thus the sample surface inspection apparatuswith high accuracy in the semiconductor device manufacturing process canbe supplied.

In addition, in the manufacturing of the semiconductor device, thedetection of the overlay can be accomplished quickly yet with highaccuracy and thus the defect inspection apparatus with high accuracy inthe semiconductor device manufacturing can be supplied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an apparatus embodying a surfaceinspection method of the present invention;

FIG. 2 shows the main component of FIG. 1 in detail;

FIG. 3 is a diagram illustrating an embodiment of a surface inspectionapparatus equipped with a plurality of preparatory environmentcompartments;

FIG. 4 is a flow chart for controlling a substrate voltage or aretarding voltage;

FIG. 5 is another flow chart for controlling a substrate voltage or aretarding voltage;

FIG. 6 shows another example of a blanking signal;

FIG. 7 shows a relationship among a substrate voltage or a retardingvoltage and an EB-CCD and a blanking signal;

FIG. 8 shows a diagram for illustrating an image formation condition fora secondary optical system;

FIG. 9 is a conceptual diagram of an overlay;

FIG. 10 is a conceptual diagram illustrating an inspection area;

FIG. 11 is a conceptual diagram of an overlay mark arrangement;

FIG. 12 is a schematic diagram of an example of an apparatus embodying asurface inspection method of the present invention;

FIG. 13 shows a pattern of an overlay;

FIG. 14 is a conceptual diagram illustrating a movement of a stage;

FIG. 15 is a plot for illustrating a theory of an overlay image-taking;

FIG. 16 is a drawing representing a conceptual diagram of image-takingwith a time difference;

FIG. 17 shows another example of the present invention;

FIG. 18 shows yet another example of the present invention;

FIG. 19 is a diagram illustrating a principle of electron beamformation;

FIG. 20 is a conceptual diagram of a blanking operation;

FIG. 21 is a conceptual diagram of a blanking direction;

FIG. 22 depicts an irradiation size of an electron beam and animage-taking concept;

FIG. 23 depicts an irradiation size of an electron beam and animage-taking concept when a stage is driven to make a turning motion;and

FIG. 24 shows an example of a pattern distribution for investing in lensaberration research.

LIST OF REFERENCE NUMERALS

1, 1 a Sample inspection apparatus

2, 2 a Primary optical system

3 Secondary optical system

4 Detection system

5, 5 a Stage unit

12 Chamber

21, 21 a Electron gun

23 ExB filter

41, 41 a Detector

42 Storage unit

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of a method for inspecting a sample surface for anydefects or the like according to the present invention will be describedbelow.

First, referring to FIG. 1, an entire apparatus for implementing theembodiment of a surface inspection method for inspecting a samplesurface for any defect is shown with reference numeral 1. In FIG. 1,reference numeral 2 designates a primary electro-optical system(hereinafter, simply referred to as a primary optical system), 3 asecondary electro-optical system (hereinafter, simply referred to as asecondary optical system), 4 a detection system, 5 stage unit disposedon a vibration isolation bed having a known structure, all of which arecontained in a housing 11 defining a chamber 12. The chamber 12 isconstructed such that it can be controlled to have a desired atmosphere,a vacuum atmosphere, for example, by a unit which is not shown.

A sample “W”, such as a wafer or a substrate, for example (the followingdescription of the present embodiment is directed to an example usingthe wafer as the sample) can be securely but removably placed on a waferholding table 51 in the stage unit 5 having a known structure andfunction by a known means, such as an electrostatic chuck, for example.The wafer holding table 51 is configured to move serially or in astep-and-repeat manner in at least one direction of two orthogonal axialdirections, or X and Y directions. A vibration proofing structure of thevibration isolating bed may be formed from a non-contacting bearing.

As shown in detail in FIG. 2, an electron gun of the primary opticalsystem for irradiating a primary electron beam may use an electron gunof a thermionic emission type or a Schottky type. The primary electronbeam “B1” emitted from the electron gun 21 will have its configurationshaped properly via a quadruple lens 22 and the like of the primaryoptical system and then irradiated onto the surface of the sample or thewafer W placed on the wafer holding table 51. In this stage, the primaryelectron beam is guided through an ExB filter or Wien filter 23comprising an electric field and a magnetic field to the wafer surface.The geometry of the primary electron beam emitted from the electron gun21 of the primary optical system 2 may be shaped such that it isirradiated onto the sample surface with uniform distribution to anextent larger than an area corresponding to pixels of a TDI-CCD or a CCDconstructing a detector 41 of the detection system 4.

Secondary electrons “B2” are generated from the surface of the wafer Win response to the irradiation of the primary electron beam, by anamount corresponding to energy of the primary electron beam. Thosesecondary electrons are accelerated by an electrode located adjacent tothe wafer toward the defector side until the secondary electrons have apredetermined amount of kinetic energy. The accelerated secondaryelectrons B2 go straight through the ExB filter or Wien filter 23comprising the electric field and the magnetic field as described above,and are guided to the secondary electro-optical system (hereinaftersimply referred to as the secondary optical system) 3. In this stage,the wafer surface could have been charged by the irradiation of theprimary electron beam, and consequently the secondary electrons mayoccasionally fail in acceleration to the predetermined amount of kineticenergy. In the event of such a failure, the secondary electrons couldnot be successfully formed into an image on a detection plane of thedetector 41, resulting in no image obtained or an unfocused image. Toaddress this, a charge amount from the electron beam irradiation overthe wafer surface should be previously calculated, and the samplevoltage or the retarding voltage should be modified adaptively independence on the calculated charge amount. This enables the secondaryelectrons to be accelerated to the predetermined amount of kineticenergy by taking the amount of charging from the electron beamirradiation into account.

Secondary electrons are formed into an image on the detector 41 as a mapprojection image by the secondary optical system 3. The electric lens orelectrostatic lens 31, a component of the secondary optical system 3,comprises plural sheets of coaxially located electrodes having aperturesor a plurality of electrode groups disposed coaxially, wherein a numberof thus configured lenses are disposed in multi-level. The electric lensserves to enlarge image data possessed by the secondary electrons, whileguiding it to the detector as map projection data so as not to loseposition and surface data on the wafer W.

The detector 41 may comprise a MCP (Multi Channel Plate) in conjunctionwith a fluorescent screen and a TD-CCD or EB-CCD or EB-TDI. Theelectrons multiplied by the MCP are then converted to light in thefluorescent screen, which light signal is taken by the TDI-CCD andoutput as an image signal. Alternatively, the secondary electrons may bedirectly introduced into the EB-CCD for converting into the imagesignal.

It is to be noted that each of the components of the primary and thesecondary optical system as well as the detecting system may have aknown structure and function, and so any further description should beherein omitted.

The stage unit 5 for holding the wafer W may have a structure to providea serial movement, if the detector is the TDI-CCD or the EB-TDI.Further, the stage is structured not only to make a serial movement butalso to repeat a go-and-stop motion in case of the detector implementedby the TDI-CCD or the EB-TDI.

If the detector is the CCD or the EB-CCD, the stage is also allowed torepeat the go-and-stop motion.

The position of the stage is always measured by a laser interferometer,though not shown, in a known method, and a current value of the positiongiven by the measurement from the laser interferometer is compared to apredetermined target value, and based on a resultant residual error, asignal for correcting the residual error is sent to an electrostaticlens control unit (not shown) of the secondary optical system 3. Acorrection mechanism is provided, in which a moving and stopping motionor a speed fleck and minute vibration during these motions may becorrected by modifying the path of the secondary electrons by means ofthe electrostatic lens as described above, so that a stableimage-forming condition can be always provided on the detection plane ofthe detector. The stage unit is provided with a brake system (notshown), and the brake system may be used upon stopping of the stage soas to reduce or even eliminate any minute vibration during stoppingmotion.

If the detector is the TDI or the EB-TDI, the apparatus has such afunction that the moving distance of the stage is measured by the laserinterferometer, and the image data taken by the TDI or the EB-TDI may beforwarded each time the stage is moved by a predetermined distance.

Electric image data obtained by the detector 41 is stored in a storageunit 42. The storage unit 42 is contained in a control section forcontrolling the TDI-CCD so as to synchronize the timing for controllingthe TDI-CCD with the timing for storing data. The image signal is inputby a known method to an image processing unit though not shown, wheresignal processing or image analysis is carried out in a known method toidentify the location of defect and determine the type of defect, andthe result may be notified to an observer, while at the same time beingstored in a storage media.

For the overlay inspection, a shift length in an orthogonal twodirections or in the X and Y directions and a shift amount in arotational angle (θ) between the under layer pattern formed in advanceand the upper layer pattern formed thereon are calculated from the imageanalysis to determine whether the overlay is right or wrong.

For the defect inspection, a cell-to-cell inspection for making acomparison between patterns in a repeated pattern arrangement or adie-to-die inspection for making a comparison between images throughpattern matching by every die may be applied. Alternatively, adie-to-any die inspection for making a comparison of one die to othermany dies or a die-to-CAD data inspection for making a comparison of onedie to a predetermined pattern in the specific design may be applied. Todetermine whether a defect exists or not, a difference is determinedrelative to the comparative image and a site with a larger differenceshould be considered defective.

Further, for the defect inspection, a physical defect in an oxide filmtranscription pattern or in a wiring, an electrical defect ofshort-circuit and an electrical defect of open-circuit, such aspotential contrast or voltage contrast, can be also detected. The itemsto be inspected may be a product of wafer, a TEG (Test Element Group), areticle or a mask.

The inspection may be selectively performed in an on-line or off-lineinspection, and it is also possible in the on-line inspection method toprovide a feedback of an inspection result directly to a semiconductormanufacturing line as an electric signal or the like via a signal line.Further, it is also possible in the off-line inspection method that theinspection result is directly input from a terminal of the inspectionapparatus to provide a feedback thereof to the semiconductormanufacturing line as an electric signal or the like via a signal line.The inspection result may be used for quality control in the course ofmanufacturing process via a communication with a host computer in thesemiconductor manufacturing line.

With reference to FIG. 1, the description is now directed to anoperation for loading the wafer W as before the inspection onto thestage unit 5 within the chamber 12 and unloading the wafer W as afterthe inspection from the stage unit.

A preparatory environment compartment 62 located adjacent to the chamber12 of the sample surface inspection apparatus 1 is configured such thatin the semiconductor manufacturing process, an environment associatedwith the wafer carried in from the outside is altered to an environmentexisting inside the chamber 12 where the stage unit 5 with the waferholding table 51 is located, until the environment within thepreparatory environment compartment 62 is in conformity with theenvironment inside the chamber 12 to allow the wafer as before theinspection to be carried in from the preparatory environment compartment62 onto the wafer holding table.

Specifically, a gate valve 63 is disposed between the housing 11 and ahousing 61. The housing 11 defines the chamber 12 containing thevibration isolating bed having a known vibration proofing structure, andthe stage unit 5 having the wafer holding table 51. The stage unit 5 isdisposed on the vibration isolating bed. The housing 61 defines thepreparatory environment compartment 62. The chamber 12 and thepreparatory environment compartment 62 can be selectively placed incommunication with each other or blocked from each other, via the gatevalve 53. In addition, another gate valve or flange may be arranged inorder to introduce, into the preparatory environment compartment, thewafer in the chamber and the preparatory environment compartment 62. Inthis regard, when the wafer is transferred between the preparatoryenvironment compartment 62 and the chamber 12 through the gate valve 63,the environments inside both of the chamber and the compartment are keptsubstantially equal (e.g., in a vacuum atmosphere at a degree of vacuumaround 10⁻⁴ Pa to 10⁻⁶ Pa).

Since in the semiconductor manufacturing process, the wafer subject tothe inspection before being transferred to a subsequent step is held inan environment suitable for a transfer to the subsequent step, thepreparatory environment compartment is firstly controlled to achieve theenvironment for transferring the wafer to the subsequent step in a knownmanner by means of a gas supply unit (not shown) and a vacuum evacuationunit, both having a known structure. Once the environment fortransferring the wafer to the subsequent step and the environment insidethe preparatory environment compartment (vacuum condition) have becomeequal, the another valve or flange operable to introduce the wafer intothe preparatory environment compartment is opened to allow the wafer tobe introduced into the preparatory environment compartment 62, where thevacuum evacuation system or the gas supply unit as mentioned above iscontrolled to now achieve the same environment (vacuum condition) as theenvironment in which the wafer holding table 51 is located or inside thechamber 12.

After that, the gate valve 63 for isolating the chamber 12 from thepreparatory environment compartment 62 is opened to allow the wafer W asbefore the inspection to be transferred onto the wafer holding table 51(this step referred to as loading). After the transfer of the wafer asbefore the inspection having been completed, the gate valve 63 isclosed, and the environment in which the wafer holding table is locatedis adjusted to be suitable for the inspection and then the inspection isstarted.

When the wafer having finished with the inspection is carried out of thewafer holding table 51 (the operation referred to as unloading) andtransferred to the subsequent step, the operation may be carried out inan inverse order to the loading. In this regard, preferably the vacuumevacuation unit may be implemented by a combination of a turbo-molecularpump 66 with a dry-root pump 67, but a rotary pump equipped with an oilmist trap or a molecular sieve may be used instead of the dry-rootspump.

FIG. 3 shows an embodiment comprising a plurality (two in theillustrated embodiment) of preparatory environment compartments 62. Theloading and the unloading operations of the wafer to be inspected may becarried out concurrently in a parallel manner. In addition, thepreparatory environment compartment may have a function for storing astock consisting of a plurality of wafers at one time. In this case, thenumber of operations for opening the gate valve may be reduced and so anefficient inspection as well as loading and unloading operations can beachieved.

FIG. 4 shows a control flow of a wafer voltage (also referred to as asubstrate voltage, a sample voltage or a retarding voltage, whereascollectively referred to as the wafer voltage for the purport ofclarity). The flow as illustrated in FIG. 4 represents the flow inone-shoot image-taking in the Still-mode with the CCD or the EB-CCD orthe TDI-CCD. The wafer voltage (the substrate voltage, the samplevoltage or the retarding voltage) is referred to as a voltage that ispreviously applied to a sample, such as a wafer and a substrate.

This represents the embodiment in which a Dose amount (referred to as adosage of the electron beam, representing an amount of irradiatedcharges as per a unit area over a sample such as a wafer and asubstrate, hereinafter referred to as the Dose amount) is calculatedfrom a signal of current density and a blanking signal so as to controlthe EB-CCD by using the blanking signal.

A current density “J_(e)” can be computed from an electron current valueof an electron gun. The Dose amount for the wafer surface can becalculated from the current density J_(e) and a blanking cancellationtime “τ_(S)” of the blanking signal.

-   -   Wherein, Dose amount=J_(e)·τ_(S)

The electrostatic capacity “C” as per unit area in the sample surface,or the wafer surface, can be determined from the data on the wafersurface, for example, a resist thickness “d” and a relative dielectricconstant “∈_(r)”.

-   -   Wherein, C=∈_(r)·∈₀/d (unit area is calculated by cm², ∈₀ is a        dielectric constant in vacuum)

In addition, from CV=Q,

a variation in wafer surface voltage ΔV=Q/C,

wherein, the wafer surface voltage may be also referred to as asubstrate surface voltage or a sample surface voltage, representing avoltage determined by summing up (superposing) an originally appliedwafer voltage and a voltage applied through the irradiation of theelectron beam to the wafer.

On the other hand, the “Q” represents a total amount of electronsirradiated to the wafer surface and assuming that a secondary electronemission rate is denoted by “γ” at the time of landing energy “LE”(keV), thenQ=Dose amount·(1−γ)=J _(e)·τ_(S)·(1−γ)

Therefore, the variation in wafer surface voltage can be expressed asfollows:ΔV=J _(e)·τ_(S)·(1−γ)·d/∈ _(r)·∈₀

Accordingly, the wafer voltage (or the retarding voltage) RTD should beadjusted to satisfyRTD+ΔV=design value (secondary electron drawing voltage).

FIG. 5 shows another control flow of the wafer voltage.

This represents an embodiment for a case where the blanking signal isdetermined based on the signal from EB-CCD, and the Dose amount isdetermined from the blanking signal and a current density signal.

FIG. 6 illustrates a relationship among the wafer voltage, the EB-CCDand the blanking signal, when one-shoot image-taking in the Still-modewith the CCD or EB-CCD or TDI-CCD is performed serially by a number oftimes. Since the Dose amount varies at each image-taking operation,therefore the wafer voltage (the retarding voltage) must be adjusted ineach case. That is to say, the same image can be always obtained byadjusting the wafer voltage in each case, and integration is applied tothose images to obtain an image with high S/N ratio and thus to improvethe precision during image analysis.

It is to be noted that the integration may be repeated by any times asdesired. Specifically, an optimal number of times of integration may beset according to the specific conditions of the wafer. In this way, theinspection can be carried out under an optimal inspection conditionaccording to the specific wafer.

FIG. 7 shows another embodiment of the blanking signal. In thisembodiment, since a blanking cancellation would occur by a number oftimes during an exposure period to the EB-CCD, the variation in wafersurface voltage ΔV can be expressed as follows:ΔV=J _(e)·Σ(τ_(S))·(1−γ)·d/∈ _(r)·∈₀

In this way, the image can be obtained by adjusting the Dose mount suchthat the sum of the wafer voltage and the variation in wafer surfacevoltage can satisfy the image formation condition for the secondaryoptical system. The exposure period and the blanking cancellation periodcan be determined relatively as desired. Specifically, the blankingcancellation period may be longer than the exposure period. In thiscase, to calculate the Dose amount, the exposure time may be substitutedfor the τ_(S).

Referring now to FIG. 8, a specific image forming condition of thesecondary optical system will be described. A primary electron beamgenerated by an electron gun is irradiated onto a surface of a wafer (ora substrate) prepared as a sample via a primary optical system (notshown in FIG. 8). Secondary electrons emanate from the wafer surface inresponse to the irradiation of the electron beam. Those secondaryelectrons are guided to a secondary optical system by using acombination of a wafer voltage (or a retarding voltage) with a voltageby an electrode located in the secondary optical system. In this step,the secondary electrons are guided so as to satisfy an image formingcondition as determined previously in the specific design and thusformed into an image on a detection plane of a detector represented byan EB-CCD.

If the potential in the surface of the wafer varies due to theirradiation of the electron beam, the combination of the wafer voltagewith the voltage by the electrode located in the secondary opticalsystem could not satisfy the image forming condition as determinedpreviously in the design, and consequently the secondary electrons cannot be formed into an image on the detection plane of the detector.

To address that, an amount of potential in the wafer surface that wouldbe varied in dependence on the irradiation of the electron beam may bepreviously superposed on the combination of the wafer voltage with thevoltage by the electrode located in the secondary optical system.

The primary electron beam generated by the electron gun is irradiatedonto the surface of the wafer (or the substrate) prepared as the samplevia the primary optical system (not shown in FIG. 8). During this step,the electron beam is irradiated concurrently to a plurality of patternsformed on the substrate consisting of at least two different types ofmaterials or at least two different types of sectional structures.Further, the electron beam is irradiated onto an area larger than afield of view in a mapping optical system. From this wafer surface, thesecondary electrons emanate in response to the irradiation of theelectron beam. Those secondary electrons are guided to the secondaryoptical system by means of a combination of the wafer voltage (or theretarding voltage) with the voltage by the electrode located in thesecondary optical system. During this step, the secondary electrons areguided so as to satisfy the image forming condition as determinedpreviously in the specific design and formed into an image on thedetection plane of the detector represented by the EB-CCD. In this way,when the plurality of patterns formed on the substrate consisting of atleast two different types of materials or two different types ofsectional structures are irradiated concurrently, the different types ofmaterials or the different types of sectional structures have differentamounts of charge-up from one another, and so if the substrate voltageor the retarding voltage is set to the specific charge-up amount inconformity with either one of the materials or sectional structures,then the contrast between the different types of materials or sectionalstructures on the substrate can be enhanced for image formation.

Further, by providing the irradiation of the electron beam onto the arealarger than the field of view of the mapping optical system, thesymmetry of the image in the X- and Y-directions can be ensured and thusan enlarged image representing realistically an actual image (an actualpattern) can be obtained.

Although before the irradiation of the electron beam, the combination ofthe wafer voltage (or the retarding voltage) of the secondary opticalsystem with the voltage by the electrode located in the secondaryoptical system is not in conformity with the image forming condition asdetermined previously in the design, as a change occurs in the potentialin the wafer surface by the irradiation of the electron beam, thesecondary electrons are due to satisfy the image forming condition forthe secondary optical system as determined previously in the design andthus can be formed into an image on the detection plane of the detector.

The combination of the wafer voltage with the voltage by the electrodelocated in the secondary optical system may be set as desired independence on the specific type of the sample, such as the substrate andthe wafer or the specific material of the surface of the sample inconjunction with a current value or a current density or an energy ofthe electron beam.

The combination of the wafer voltage with the voltage by the electrodelocated in the secondary optical system may be set so that the secondaryelectrons, after a number of times of irradiation, can satisfy the imageforming condition for the secondary optical system as determinedpreviously in the design.

The combination of the wafer voltage with the voltage by the electrodelocated in the secondary optical system may be set such that for eachirradiation of the electron beam, the secondary electrons may satisfythe image forming condition for the secondary optical system asdetermined previously in the design, so that when the electron beam isirradiated by a number of times, the combination of the wafer voltagewith the voltage by the electrode located in the secondary may becontrolled for each irradiation of the electron beam such that thesecondary electrons can satisfy the image forming condition for thesecondary optical system as determined previously in the design. In thiscase, the images obtained during each time of irradiation may be summedup.

When a scanning image is taken by scanning the stage or the electronbeam, the combination of the substrate voltage or the retarding voltagewith the voltage by the electrode located in the secondary opticalsystem may be controlled in response to the current density or currentvalue of the electron beam and the scanning speed of the stage orelectron beam so that the secondary electrons can satisfy the imageforming condition for the secondary optical system as determinedpreviously in the design.

An embodiment of an overlay inspection method will now be described.

First referring to FIG. 9, there is shown a conceptual diagramillustrating an overlay inspection. In FIG. 9, reference numeral 100designates a silicon substrate, 101 an oxide film layer, 102 an underlayer pattern, 103 a deposition film layer, and 104 a resist layer afterhaving been exposed to a light and then developed. A semiconductormanufacturing process involves a number of etching processes. An etchingprocess provides the steps of applying a resist over a deposition filmto be desirably etched, for example, the oxide film 103; exposing theresist to a light or an electron beam and then developing it so as toform a desired pattern in the resist layer 104; and etching and therebyremoving a portion of the deposition film, for example, the oxide filmthat is not covered with the resist layer so as to form it into adesired pattern.

Over the pattern 102 that has been created in the first etching process(hereinafter used to refer a pattern of an overlay mark), the step ofburying or deposition of a new film is applied, and thus formed filmwill again need to be processed by etching. In this stage, the pattern(an under layer pattern) 102 that has been created in the previousprocess and a pattern (an upper layer pattern) 105 that will be newlycreated by etching must be in conformity to each other in accordancewith a design. To address this, a mark for alignment is used to inspectthe conformity between the under layer pattern 102 and the upper layerpattern 105.

Since the resist has been already applied over the under layer patternfor the etching of the upper layer pattern, the under layer patternneeds to be viewed or observed through the resist. Further, theinspection of the overlay requires that the upper layer pattern and theunder layer pattern must be viewed or observed simultaneously.

In the overlay inspection, primarily the under layer pattern may oftenreside beneath the resist or the oxide film. Occasionally, it may residebeneath a conductive layer. Primarily, the upper layer pattern may beformed by exposing the resist to a light, which can be accomplished bythe exposure only, or by the steps up to the post-baking or up to thedevelopment.

FIG. 10 shows a conceptual diagram of an inspection area. The overlayinspection may be applied to a limited number of dies, such as D1 to D8in FIG. 2, for example, but not to every one of the dies. Consequently,in order to reduce the time required for travelling between dies to beinspected, the stage should be accelerated up to its maximum speed fortravelling between the dies to be inspected.

Referring now to FIG. 11, there is shown a conceptual diagram of anoverlay mark arrangement. The overlay mark can be occasionally arrangedin each die in such a configuration as shown in FIG. 11. The inspectionis occasionally limited to certain marks but not applied to every one ofthe overlay marks. Therefore, in order to reduce a time required fortravelling between the marks, the stage is accelerated to a maximumspeed for travelling between the marks.

Referring now to FIG. 12, an entire apparatus for implementing a surfaceinspection method for inspecting a sample surface for any defect or thelike on a sample surface according to the present embodiment isdesignated by reference numeral 1. Since the apparatus of the presentembodiment is similar to that of FIG. 1 except that a computer iscoupled to both of the stage control unit and the storage unit 41,description on the structure and the operation of common parts is hereinomitted.

A sample “W”, such as a wafer or a substrate, for example (the followingdescription of the present embodiment is directed to an example usingthe wafer as the sample) may be securely but removably placed on a waferholding table 51 in the stage unit 5 having a known structure andfunction by a known means, such as an electrostatic chuck, for example.The wafer holding table 51 is configured to move serially or in astep-and-repeat manner in at least one direction of two orthogonal axialdirections, or X and Y directions. A vibration proofing structure of thevibration isolating bed may be formed from a non-contacting bearing.

As shown in detail in FIG. 2, an electron gun 21 of the primary opticalsystem for irradiating a primary electron beam may use an electron gunof a thermionic emission type or a Schottky type. It is to be noted thatthe electron gun 21 may be separate from the components of the primaryoptical system. The primary electron beam “B1” emitted from the electrongun 21 will have its configuration shaped properly via a quadruple lens22 and the like of the primary optical system and then irradiated ontothe surface of the sample or the wafer W placed on the wafer holdingtable 51. In this stage, the primary electron beam is guided through anExB filter or a Wien filter 28 comprising an electric field and amagnetic field to the wafer surface.

An electron beam may be shaped by the lens of the primary optical systemsuch that a size of an irradiation area on the sample is larger thanthat of a pattern in the sample surface, especially the pattern size ofthe overlay pattern. Further, the electron beam is shaped such that ithas substantially a circular or elliptical shape and it has generallyuniformly distributed beam intensity. The electron beam is irradiatedsubstantially onto a center of the overlay mark. The irradiation of theelectron beam onto the sample surface is provided by a blankingelectrode (not shown) located in the middle of the primary opticalsystem 2. When the electron beam is to be irradiated onto the samplesurface, the voltage at the electrode is set to 0V (zero volt) or to avoltage level required to control the path of the electron beam, and theelectron beam is advanced substantially centrically through the primaryoptical system. If the electron beam is not intended to irradiate thesample surface, a sufficient voltage to divert the electron beamcompletely out of the primary optical system is applied to the blankingelectrode so as to guide the electron beam to an outer wall constitutingthe primary optical system or a specialized electrode (not shown) or thelike to achieve blanking for preventing the electron beam from beingirradiated onto the sample surface.

FIG. 13 shows an overlay mark or an overlay pattern. The overlay markmay employ a bar-in-bar type or a bar-in-box type pattern. The outerbars represent an under layer pattern below a resist layer and the innerbars or box represent the resist pattern, which may have been undergonethe steps up to exposure, exposure and PEB (preheating) or up todevelopment. The under layer pattern may be an STI structure or may be ametal wiring or a trench structure.

Secondary electrons “B2” are generated from the surface of the wafer Win response to the irradiation of the primary electron beam, by anamount corresponding to energy of the primary electron beam. Thosesecondary electrons are accelerated by an electrode located adjacent tothe wafer toward the defector side until the secondary electrons have apredetermined amount of kinetic energy. The accelerated secondaryelectrons B2 go straight through the ExB filter or Wien filter 28comprising the electric field and the magnetic field as described above,and are guided to the secondary electro-optical system (hereinaftersimply referred to as the secondary optical system) 3. In this stage,the wafer surface could have been charged by the irradiation of theprimary electron beam, and consequently the secondary electrons mayoccasionally fail in acceleration to the predetermined amount of kineticenergy. In the event of such a failure, the secondary electrons couldnot be successfully formed into an image on a detection plane of adetector 41, resulting in no image obtained or an unfocused image. Toaddress this, a charging amount from the electron beam irradiation overthe wafer surface should be previously calculated, and a sample voltageor a retarding voltage should be modified adaptively in dependence onthe calculated charging amount. This enables the secondary electrons tobe accelerated to the predetermined amount of kinetic energy by takingthe amount of charging from the electron beam irradiation into account.

Secondary electrons are formed into an image on the detector 41 as a mapprojection image by the secondary optical system 3. The electric lens orelectrostatic lens 31, a component of the secondary optical system 3,comprises plural sheets of coaxially located electrodes having aperturesor a plurality of electrode groups disposed coaxially, wherein a numberof thus configured lenses are arranged in multi-level. The electric lensserves to enlarge image data possessed by the secondary electrons, whileguiding it to the detector as map projection data so as not to loseposition and surface data on the wafer W.

The detector 41 may comprise a MCP (Multi Channel Plate) in conjunctionwith a fluorescent screen and a TD-CCD or EB-CCD or EB-TDI. Theelectrons multiplied by the MCP are then converted to light in thefluorescent screen, which light signal is taken by the TDI-CCD andoutput as an image signal. Alternatively, the secondary electrons may bedirectly introduced into the EB-CCD for converting into the imagesignal.

It is to be noted that each of the components of the primary and thesecondary optical system as well as the detecting system may have aknown structure and function, and so any further description should beherein omitted.

The stage unit 5 for holding the wafer W may have a structure to providea serial movement if the detector is the TDI-CCD or the EB-TDI. Further,the stage is structured not only to make a serial movement but also torepeat a go-and-stop motion in case of the detector implemented by theTDI-CCD or the EB-TDI.

If the detector is the CCD or the EB-CCD, the stage is also allowed torepeat the go-and-stop motion.

The position of the stage is always measured by a laser interferometer,though not shown, in a known method, and a current value of the positiongiven by the measurement from the laser interferometer is compared to apredetermined target value, and based on a resultant residual error, asignal for correcting the residual error is sent to an electrostaticlens control unit (not shown) of the secondary optical system 3. Acorrection mechanism is provided, in which a moving and stopping motionor a speed fleck and minute vibration during these motions may becorrected by modifying the path of the secondary electrons by means ofthe electrostatic lens as described above, so that a stableimage-forming condition can be always provided on the detection plane ofthe detector. The stage unit is provided with a brake system (notshown), and the brake system may be used upon stopping of the stage soas to reduce or even eliminate any minute vibration during stoppingmotion.

The electric image data obtained by the detector 4 is input to an imageprocessing unit, though not shown, where signal processing or imageanalysis is carried out to identify the location of defect and determinethe type of defect, and the result may be notified to an observer, whilebeing stored in a storage media. For the overlay inspection, a shiftlength in the X and Y directions and a shift amount in a rotationalangle (θ) between the under layer pattern and the upper layer patternare calculated from the image analysis to determine whether the overlayis right or wrong.

The inspection may be selectively performed in an on-line or off-lineinspection, and it is also possible in the on-line inspection method toprovide a feedback of an inspection result directly to a semiconductormanufacturing line as an electric signal or the like via a signal line.Further, it is also possible in the off-line inspection method that theinspection result is directly input from a terminal of the inspectionapparatus to provide a feedback thereof to the semiconductormanufacturing line as an electric signal or the like via a signal line.The inspection result may be used for quality control in the course ofmanufacturing process via a communication with a host computer in thesemiconductor manufacturing line.

Since the operations for loading the wafer W as before the inspectiononto the stage unit 5 within the chamber 12 and unloading the wafer W asafter the inspection out of the stage unit are similar to thosedescribed above with reference to FIG. 1, the detailed description isherein omitted. It is a matter of course that a configuration comprisinga plurality (two in this embodiment) of the preparatory environmentcompartments 62 as shown in FIG. 3 may be applicable to the illustratedembodiment.

A Dose amount (referred to as a dosage of the electron beam,representing an amount of irradiated charges as per a unit area over asample such as a wafer and a substrate, hereinafter referred to as theDose amount) is calculated from a signal of current density and ablanking signal, and the EB-CCD is controlled by using the blankingsignal.

The theory for controlling the RTD voltage or the substrate voltage willbe as follows, similarly to that in the embodiment above. A currentdensity “J_(e)” can be computed from an electron current value of anelectron gun. The Dose amount for the wafer surface can be calculatedfrom the current density J_(e) and a blanking cancellation time “τ_(S)”of the blanking signal.

Wherein, Dose amount=J_(e)·τ_(S)

The electrostatic capacity “C” as per unit area in the sample surface,or the wafer surface, can be determined from the data on the wafersurface, for example, a resist thickness “d” and a relative dielectricconstant “∈_(r)”.

Wherein, C=∈_(r)·∈₀/d (unit area is calculated by cm², ∈₀ is adielectric constant in vacuum)

In addition, from CV=Q,

a variation in wafer surface voltage ΔV=Q/C,

wherein, the wafer surface voltage may be also referred to as asubstrate surface voltage or a sample surface voltage, representing avoltage determined by summing up (superposing) an originally appliedwafer voltage and a voltage applied through the irradiation of theelectron beam to the wafer.

On the other hand, the “Q” represents a total amount of electronsirradiated to the wafer surface and assuming that a secondary electronemission rate is denoted by “γ” at the time of landing energy “LE”(keV), thenQ=Dose amount·(1−γ)=J _(e)·τ_(S)·(1−γ)

Therefore, the variation in wafer surface voltage can be expressed asfollows:ΔV=J _(e)·τ_(S)·(1−γ)·d/∈ _(r)·∈₀

Accordingly, the wafer voltage (or the retarding voltage) RTD should beadjusted to satisfyRTD+ΔV=design value (secondary electron drawing voltage).

FIG. 14 shows a conceptual diagram of a motion of the stage. Thedescription will be given to the stage motion in a case where the EB-TDIor the TDI is employed as the detector for image-taking operation. Thestage is accelerated up to a maximum speed until it reaches the positionof the overlay mark as determined previously, and then in the overlaymark region, the stage is moved at a speed in synchronism with anoperating frequency of the EB-TDI or the TDI for taking an image of theoverlay mark. Further, when moving to another overlay mark, the stage ismoved while being accelerated.

A step-and-repeat motion is performed for taking an image by the EB-CCDor the CCD. It is to be noted that to take an image of the overlay mark,the same overlay mark may be used repeatedly by a number of times forimage-taking.

For the overlay inspection, there may be a case where the condition todetermine the ΔV for viewing the under layer pattern is different fromthe condition to determine the ΔV for viewing the upper layer pattern.Since in this case, it is impossible to obtain the upper layer patternand the under layer pattern at the same time, the image taking operationis carried out a number of times to thereby obtain the lower layerpattern and the upper layer pattern separately, and those images arecombined to form a synthetic image, from which any misalignment betweenthe under layer pattern and the upper layer pattern may be detected orcalculated. In this operation, the same pattern may be repeatedly usedfor image-taking. Alternatively, the repeated image taking may beapplied to each die repeatedly or may be applied to each waferrepeatedly. To do this, preferably the conditions for obtaining animage, especially the RTD and the Dose amount, may be set to theconditions suitable for the under layer pattern.

Especially, there may be a case where the sectional structure or thematerial of the substrate surface and thus a time period for chargingand a time period for the charges in the surface to escape (dischargingperiod) are different from each other in dependence on the specificprocess. In such a case, even if a single pattern is subject toimage-taking, the pattern may be subject to image-taking with a timedifference rather than repeated serial operations.

Turning now to FIGS. 15(A) and 15(B), the theory of the overlayimage-taking will be described. In FIGS. 15(A) and 15(B), respectively,the vertical axis indicates a potential in the surface of the sample orthe wafer and the horizontal axis indicates a time elapsed since thebeginning of the electron beam irradiation.

The description is herein directed to the image-taking by a singleirradiation as shown in FIG. 15(A). If an amount of irradiation ofelectrons is increased for the image-taking to be accomplished by asingle irradiation, the potential in the wafer surface could rise to V6within a charging period determined in dependence on a feature of thewafer surface. Assuming that the condition for forming the secondaryelectrons from the wafer surface into an image by adjusting the surfacepotential and the RTD is V3, then in the single image-taking operationas shown in FIG. 15(A), the surface potential of the wafer would exceedthe level of the image forming condition before the image being taken,resulting in only an unfocused image or no image obtained.

Further, if the amount of irradiation (i.e., dose) of electrons isdecreased in order to make the final surface (V6) to V3 by beingcharged, an amount of secondary electrons is decreased, and accordinglya resultant image would be dark or no image would be obtained.

In contrast to that, when the irradiation of electron beam is givenlittle by little with some interval between irradiations, as shown inFIG. 15(B), the wafer surface potential can be discharged during theinterval between respective irradiations, so that the potential in thewafer surface can be controlled by taking advantage of the charging anddischarging from the irradiation to thereby make it possible to take animage at a specific timing that can produce V3 representing the imageforming condition.

The above approach is illustrated in a conceptual diagram in FIG. 16.This shows an example of an operation for such a substrate that requiresa longer time for discharging so that the surface potential is noteasily attenuated, in which if the repeated image taking operations arecarried out continuously, the surface potential would vary quickly andsignificantly and thus the RTD adjustment could not provide sufficienteffect to satisfy the image forming condition of the secondary EO(Electro-Optic) system or of the secondary optical system which providesthe image forming condition of the secondary electrons on the detectorsurface.

After taking an image of pattern (i.e., a pattern of an overlay markformed in a die, which will be used in the reference) 1 in a die 1 by asingle time, the operation process is moved to a pattern 2 in a die 2for taking an image thereof by a single time, and then moved to apattern 3 in a die 3 for taking an image thereof by a single time,followed by taking images of a plurality of dies including die 4, a die5 . . . and a die n, each by a single time, and after a series ofimage-taking of every die of n dies each by single time having beencompleted, the operation process returns back to the pattern 1 of thedie 1 for a second image-taking.

Each second image-taking is applied to the die 2, the die 3 . . . andthe die n, and in this way, this operation is repeated by m times asrequired.

In this regard, any misalignment between the upper layer pattern and theunder layer pattern may be calculated from a synthetic image from theimage taken in the first image-taking and the image taken in the secondimage-taking, in which the images to be used to form the synthetic imagemay be those taken at any time of the image-taking operation as desired.

Although FIG. 16 shows an example where the image-taking operation isconducted while moving from one die to another die, if images of aplurality of patterns within a single die are to be taken, then aplurality of image-taking operations may be carried out while movingfrom one pattern to another pattern within the die. Again in this case,any misalignment between the upper layer pattern and the under layerpattern may be calculated from a synthetic image from the image taken inthe first operation and the image taken in the second operation, inwhich the images to be used to form the synthetic image may be thosetaken at any time of image-taking operation as desired.

As shown in FIG. 16, if the image-taking of one pattern is repeated by anumber of times, as moving from one pattern to another pattern,preferably the apparatus may have a function for controlling a speed ofthe stage so that a distance of stage movement should be minimized and atime required for moving the stage should be equal among all movements(the interval between image-taking of respective patterns should be madeconstant).

If there are a large number of patterns to be inspected, the patternsmay be organized into groups of a desired number of patterns so as toconduct the image-taking on a group-by-group basis, rather than takingimages of all patterns at once. Preferably, the n number of patternscontained in the group may be determined from a time difference (or aninterval period) required for image-taking and a moving speed of thestage, as calculated from the feature regarding the charging period andthe discharging period of the substrate.

The feature regarding the charging period and the discharging period ofthe substrate is input previously into the control unit for controllingthe movement of the stage, which will be combined with the position dataof the pattern to be inspected for calculating the condition where thedistance or time period of movement between patterns can be minimizedand the period of movement between patterns is equal among all patterns.

Further, the feature regarding the charging period and the dischargingperiod of the substrate is input previously into the control unit forcontrolling the movement of the stage, which will be combined with theposition data of the pattern to be inspected for calculating a specificnumber of patterns to be required for inspection and calculating thecondition where the inspection time can be minimized and the period ofmovement between patterns is equal among all patterns.

Specifically, the procedure will be carried out in the following steps.

First of all, the discharging period from a point of time when theelectron beam is irradiated onto the wafer until the charge amount inthe wafer surface becomes 0 or a predetermined value is measured. Then,a number or positions of the overlay marks subject to image-taking, orthe groups of overlay marks subject to image-taking are determined basedon the calculated discharging period as stated above. Specifically, asdescribed above, since in the illustrated embodiment, a contrastsuitable for image-taking can be obtained by repeating several times aseries of operations in which the irradiation of the electron beam isapplied to the overlay mark at a single location and before the chargeamount in the wafer surface becomes 0 or the predetermined value,another electron beam irradiation to the same overlay mark is carriedout, therefore it is required to adjust the time interval from the firstirradiation of the electron beam to the predetermined overlay mark tothe second irradiation of the electron beam to the same overlay mark soas not to exceed the discharging time period.

Secondly, the period for moving between overlays should be adjustedequally. This yields an equal time interval between one irradiation andanother of the electron beam to each overlay and thus the equal chargeamount in each overlay results in a homogeneous image taken for eachoverlay. To adjust the moving period, the highest moving speed of thestage is taken as a reference. In the illustrated embodiment, theoverlays to be measured are previously determined, and the moving periodtaken in the case to move the stage by the longest moving distancebetween two of those determined overlays at the highest speed has beentaken as a reference (the moving period in this case is referred to as areference moving period). Then, the stage moving speed between therespective overlays may be set such that the moving period for betweenrespective overlays should be equal to the reference moving period. If asum of the moving periods obtained in the above procedure exceeds thedischarging period, the selection of the overlays should be made again.

In another embodiment, every combination of the moving distances betweenthe overlay marks as well as the maximum moving speed of the stage arepreviously stored in the storage media in an apparatus, and thedischarging period is input through the input section in the apparatus,so that the computing section in the apparatus may execute an operationto determine a path to allow the maximum number of overlays to besubject to the image-taking operation within a range not exceeding thedischarging period of the wafer surface or the time determined bysubtracting a desired time period from the discharging period.

Further, there would be a case where the moving speed or theacceleration varies along respective axes, such as along the X- orY-axis, or depending on each specific position along the axes inaccordance with a configuration of the stage, and in such a case, thevariation should be taken into account for calculating or computing themoving period.

When the images of the upper and the under layer patterns are takenseparately from each other, the RTD should be controlled in dependenceon the Dose amount.

Irradiation of laser light may be employed as a method for controllingthe ΔV. This irradiation of the laser light can provide a more precisecontrol for a surface potential increment. The laser light is irradiatedin advance and then the electron beam is irradiated. The surfacepotential increment has been modified by a quantum effect from theirradiation of the laser light, and the surface potential increment inthe sample surface, which could not have been fine-tuned simply throughthe Dose amount control, can be now successfully tuned, so that aclearer image can be obtained.

The surface potential tuning by the irradiation of the laser light andthe adjustment of the RTD and the Dose amount can be provided bycontrolling these three factors together in a comprehensive manner andconcurrently.

Although it is possible to apply the integration by all images in caseof repeated image taking, alternatively the upper layer pattern imageand the under layer pattern image may be taken separately, wherein thelaser light irradiation amount and the potential increment in the samplesurface can be fine-tuned only when taking the under layer patternimage. Alternatively, the potential increment in the sample surface bythe irradiation of the laser light may be fine-tuned when taking theimage of the upper layer pattern, or the laser light irradiation may beprovided at any times. Further, the laser light irradiation may beprovided during no image being taken, but it is suspended or blockedduring the image taking.

Referring now to FIG. 17, a sample inspection apparatus according toanother embodiment of the present invention is generally shows byreference numeral 1 a. In the inspection apparatus 1 a, an electron gun21 a of a thermionic emission type or a Schottky type for irradiating anelectron beam onto the wafer is located right above a stage unit 5 a. Aprimary electron beam B1 emitted from the electron gun is irradiatedonto a wafer surface, while its beam configuration being shaped througha primary optical system 2 a comprising an electrostatic lens 22 a, suchas a quadruple lens and the like. The primary electron beam isirradiated onto an overlay mark, while being driven to make a scanningoperation in the X- and the Y-directions.

Same as the foregoing embodiment, the wafer may be securely butremovably placed on a wafer holding table 51 in the stage unit 5 havinga known structure and function by a known means, such as a vacuum chuck,for example. The wafer holding table 51 is configured to move seriallyor in a step-and-repeat manner in at least one direction of twoorthogonal axial directions, or X and Y directions. A vibration proofingstructure of the vibration isolating bed may be formed from anon-contacting bearing.

Secondary electrons B2 are generated from the surface of the wafer inresponse to the irradiation of the electron beam, by an amountcorresponding to energy of the primary electron beam. Those secondaryelectrons are accelerated by an adjacent electrode until the secondaryelectrons have a predetermined amount of kinetic energy and then guidedto a detector 41 a via a secondary optical system, though not shown. Inthis stage, the wafer surface could have been charged by the irradiationof the electron beam, and consequently the secondary electrons mayoccasionally fail in acceleration to the predetermined amount of kineticenergy as designed. In the event of such a failure, no image or anunfocused image could be obtained. To address this, as applied in theforegoing embodiment, a charging amount from the electron beamirradiation over the wafer surface should be previously calculated, andthe substrate voltage or the retarding voltage should be modifiedadaptively in dependence on the calculated charging amount. This enablesthe secondary electrons to be accelerated to the predetermined amount ofkinetic energy by taking the amount of charging from the electron beamirradiation into account. Processing of the image detected by thedetector may be carried out in a similar manner to that described in theforegoing embodiment, and any detailed description should be hereinomitted.

Operations for loading the wafer W as before the inspection onto thestage unit 5 a within the chamber 12 and unloading the wafer W as afterthe inspection are similar to the operations in the foregoingembodiment, and the description will be herein omitted.

FIG. 18 shows yet another example of the present invention. FIG. 18depicts a processing apparatus using a charged particle beam, in whichthe movement of the stage as shown in FIG. 16 is also applicable to acase for processing a material, such as an insulating material, that iseasily charged and associated with a problem of precision of processingarising from being charged. If the work piece is easily charged andconsequently could not be processed in a single operation with a highenergy density, then it is processed with a lower energy density. Ifthere are a number of work pieces, the process may make use of a timewaiting for a first work piece to be discharged so as to move to asecond work piece and process during this waiting period.

Although FIG. 18 shows an example of a charged particle beam, an energyparticle beam including, for example, a high-speed atomic beam, or anenergy beam including, for example, a laser, a maser and an X-ray may beused instead of the charged particle beam.

It is to be noted that since the configuration and operations of theprocessing apparatus may be similar to those in the conventional oneswith an exception of its pattern inspection apparatus and inspectionmethod according to the present invention, any detailed description onthose will be herein omitted.

Yet another embodiment of a sample surface inspection method accordingto the present invention will now be described. A conceptual diagram ofthe overlay inspection is similar to the illustration of FIG. 9 and thedescription on that will be omitted.

Further, since the apparatus used in this embodiment is similar to thatshown in FIG. 1, the description will be given again with reference toFIG. 1.

In FIG. 1, an entire apparatus for implementing a surface inspectionmethod for inspecting a sample surface for any defect and so on is shownwith reference numeral 1. In FIG. 1, reference numeral 2 designates aprimary electro-optical system (hereinafter, simply referred to as aprimary optical system), 3 a secondary electro-optical system(hereinafter, simply referred to as a secondary optical system), 4 adetection system, 5 a stage unit disposed on a vibration isolating bedhaving a known structure, all of which are contained in a housing 11defining a chamber 12. The chamber 12 is constructed such that it can becontrolled to have a desired atmosphere, a vacuum atmosphere, forexample, by a device though not shown.

A sample “W”, such as a wafer or a substrate, for example (the followingdescription of the present embodiment is directed to an example usingthe wafer as the sample) may be securely but removably placed on a waferholding table 51 in the stage unit 5 having a known structure andfunction by a known means, such as a chuck, for example. The waferholding table 51 is configured to move serially or in a step-and-repeatmanner in at least one direction of two orthogonal axial directions, orX and Y directions. A vibration proofing structure of the vibrationisolating bed may be formed from a non-contacting bearing.

The electron gun 21 of the primary optical system for irradiating aprimary electron beam may use an electron gun of a thermionic emissiontype or a Schottky type. It is to be noted that the electron gun may bea separate component from the primary optical system. The primaryelectron beam B1 emitted from the electron gun 21 is irradiated onto thesurface of the sample or the wafer W placed on the wafer holding table51 while being shaped in its configuration properly via the primaryoptical system 2 comprising an electrostatic lens, such as a quadruplelens 22 and the like.

An electromagnetic lens may be used in addition to the electrostaticlens such as the quadruple lens for shaping the electron beam. In FIG.19, there is shown a conceptual diagram for shaping the beamconfiguration by using additionally a shielding element having anaperture or an opening of a desired geometry, such as an aperture member26. The quadruple lens has been applied in advance with a voltage so asto form the beam into a desired beam size and geometry which arepredetermined. The configuration of the aperture has been also selectedto be suitable for forming the beam into the desired beam size andgeometry which are predetermined. It is also possible to provide aplurality of aperture members, each having a different configuration,which will be exchangeably used in accordance with the specific type ofwafer or inspection pattern. The voltage applied to the quadruple lensmay be also varied in dependence on the specific type of wafer orinspection pattern. Those may be controlled by a control unit forcontrolling the bean configuration, which is capable of calculating thecondition automatically to determine a combination of the voltageapplied to the quadruple lens with the configuration of the aperture.

It is possible to shape the beam configuration only with a rectangularaperture. In this case, the beam configuration is controlled by thecontrol unit operable to control the beam configuration in accordancewith the specific type of wafer or inspection pattern, in which thecontrol unit calculates a condition automatically for selecting anoptimal aperture. Further, the configuration of the aperture may be anelliptical shape in addition to the rectangular shape.

In this stage, the primary electron beam is guided through an ExB filteror a Wien filter 28 comprising an electric field and a magnetic field,to the wafer surface. The electron beam is formed into the beam sized tobe larger than the size of the pattern in the sample surface, especiallyof the overlay pattern.

The electron beam is formed to achieve a generally uniform distributionof the beam intensity. The electron beam is irradiated substantially toa center of the overlay mark. The irradiation of the electron beam ontothe sample surface is controlled by a blanking electrode 23 located inthe middle of the primary optical system 2, as shown in FIG. 20. Whenthe electron beam is to be irradiated onto the sample surface, thevoltage at the electrode is set to 0V (zero volt) or to a voltage levelrequired to control the path of the electron beam, and the electron beamis advanced substantially centrically through the primary opticalsystem. If the electron beam is not intended to irradiate the samplesurface, a sufficient voltage to divert the electron beam completely outof the primary optical system is applied to the blanking electrode 23 soas to guide the electron beam to an outer wall constituting the primaryoptical system or a specialized electrode 24 or the like to achieveblanking for preventing the electron beam from being irradiated onto thesample surface. The blanking electrode may be constructed from aquadruple electrode. The direction for deflecting the electron beam maybe either of the orthogonally crossing X direction or Y direction, orthe diagonal direction (including an X-directional component and anY-directional component). The blanking electrode 23 may comprise aquadruple electrode. FIG. 21 shows a conceptual diagram of the directionfor deflecting the electron beam. In FIG. 21, “IP” designates a patternto be inspected including an under layer pattern 102 and an upper layerpattern, and “BA” designates an irradiation range of the primaryelectron beam. Preferably, the direction for deflecting the electronbeam may be either in the heading direction of the stage or the oppositedirection thereto.

The direction for deflecting the electron beam may be any desireddirection relative to the heading direction of the stage.

Since the overlay mark or the overlay pattern used in the illustratedembodiment is similar to that shown in FIG. 13, and the explanationthereof is herein omitted.

Secondary electrons “B2” are generated from the surface of the wafer Win response to the irradiation of the primary electron beam, by anamount corresponding to energy of the primary electron beam. Thosesecondary electrons are accelerated by an electrode located adjacent tothe wafer toward the defector side until the secondary electrons have apredetermined amount of kinetic energy. The accelerated secondaryelectrons B2 go straight through the ExB filter or Wien filter 28comprising the electric field and the magnetic field as described above,and are guided to the secondary electro-optical system (hereinaftersimply referred to as the secondary optical system) 3. In this stage,the wafer surface could have been charged by the irradiation of theprimary electron beam, and consequently the secondary electrons mayoccasionally fail in acceleration to the predetermined amount of kineticenergy. In the event of such a failure, the secondary electrons couldnot be successfully formed into an image on a detection plane of adetector 41, resulting in no image obtained or an unfocused image. Toaddress this, a charging amount from the electron beam irradiation overthe wafer surface should be previously calculated to determine a beamsize and a shape thereof. This enables the secondary electrons to beaccelerated to the predetermined amount of kinetic energy by taking theamount of charging from the electron beam irradiation into account.

Secondary electrons are formed into an image on the detector 41 as a mapprojection image by the secondary optical system 3. The electric lens orelectrostatic lens 31, a component of the secondary optical system 3,comprises plural sheets of coaxially located electrodes having aperturesor a plurality of electrode groups disposed coaxially, wherein a numberof thus configured lenses are arranged in multi-level. The electric lensserves to enlarge image data possessed by the secondary electrons, whileguiding it to the detector as map projection data so as not to loseposition and surface data on the wafer W.

The detector 41 may comprise a MCP (Multi Channel Plate) in conjunctionwith a fluorescent screen and a TD-CCD or EB-CCD or EB-TDI. Theelectrons multiplied by the MCP are then converted to light in thefluorescent screen, which light signal is taken by the TDI-CCD andoutput as an image signal. Alternatively, the secondary electrons may bedirectly introduced into the EB-CCD for converting into the imagesignal.

It is to be noted that each of the components of the primary and thesecondary optical system as well as the detecting system may have aknown structure and function, and so any further description should beherein omitted.

Further, the secondary electrons after having been multiplied by the MCPmay be introduced directly into the EB-TDI. In addition, the fluorescentscreen along with the TDI may construct the detector.

The stage unit 5 for holding the wafer W may have a structure to providea serial movement if the detector is the TDI-CCD or the EB-TDI. Further,the stage is structured not only to make a serial movement but also torepeat a go-and-stop motion in case of the detector implemented by theTDI-CCD or the EB-TDI.

If the detector is the CCD or the EB-CCD, the stage is also allowed torepeat the go-and-stop motion.

The position of the stage is always measured by a laser interferometer,though not shown, in a known method, and a current value of the positiongiven by the measurement from the laser interferometer is compared to apredetermined target value, and based on a resultant residual error, asignal for correcting the residual error is sent to an electrostaticlens control unit (not shown) of the secondary optical system 3. Acorrection mechanism is provided, in which a moving and stopping motionor a speed fleck and minute vibration during these motions may becorrected by modifying the path of the secondary electrons by means ofthe electrostatic lens as described above, so that a stableimage-forming condition can be always provided on the detection plane ofthe detector. The stage unit is provided with a brake system (notshown), and the brake system may be used upon stopping of the stage soas to reduce or even eliminate any minute vibration during stoppingmotion.

The electric image data obtained by the detector 4 is input to an imageprocessing unit, though not shown, where signal processing or imageanalysis is carried out to identify the location of defect and determinethe type of defect, and the result may be notified to an observer, whilebeing stored in a storage media. For the overlay inspection, a shiftlength in the X and Y directions and a shift amount in a rotationalangle (θ) between the under layer pattern and the upper layer patternare calculated from the image analysis to determine whether the overlayis right or wrong.

The inspection may be selectively performed in an on-line or off-lineinspection, and it is also possible in the on-line inspection method toprovide a feedback of an inspection result directly to a semiconductormanufacturing line as an electric signal or the like via a signal line.Further, it is also possible in the off-line inspection method that theinspection result is directly input from a terminal of the inspectionapparatus to provide a feedback thereof to the semiconductormanufacturing line as an electric signal or the like via a signal line.The inspection result may be used for quality control in the course ofmanufacturing process via a communication with a host computer in thesemiconductor manufacturing line.

With reference to FIG. 1, the description is now directed to anoperation for loading the wafer W as before the inspection onto thestage unit 5 within the chamber 12 and unloading the wafer W as afterthe inspection from the stage unit.

A preparatory environment compartment 62 located adjacent to the chamber12 of the sample surface inspection apparatus 1 is configured such thatin the semiconductor manufacturing process, an environment associatedwith the wafer carried in from the outside is altered to an environmentexisting inside the chamber 12 where the stage unit 5 with the waferholding table 51 is located, until the environment within thepreparatory environment compartment 62 is in conformity with theenvironment inside the chamber 12 to allow the wafer as before theinspection to be carried in from the preparatory environment compartment62 onto the wafer holding table.

Specifically, a gate valve 63 is disposed between the housing 11 and ahousing 61. The housing 11 defines the chamber 12 containing thevibration isolating bed having a known vibration proofing structure, andthe stage unit 5 having the wafer holding table 51. The stage unit 5 isdisposed on the vibration isolating bed. The housing 61 defines thepreparatory environment compartment 62. The chamber 12 and thepreparatory environment compartment 62 can be selectively placed incommunication with each other or blocked from each other, via the gatevalve 53. In addition, another gate valve or flange may be arranged inorder to introduce, into the preparatory environment compartment, thewafer in the chamber and the preparatory environment compartment 62. Inthis regard, when the wafer is transferred between the preparatoryenvironment compartment 62 and the chamber 12 through the gate valve 63,the environments inside both of the chamber and the compartment are keptsubstantially equal (e.g., in a vacuum atmosphere at a degree of vacuumaround 10⁻⁴ Pa to 10⁻⁶ Pa).

Since in the semiconductor manufacturing process, the wafer subject tothe inspection before being transferred to a subsequent step is held inan environment suitable for a transfer to the subsequent step, thepreparatory environment compartment is firstly controlled to achieve theenvironment for transferring the wafer to the subsequent step in a knownmanner by means of a gas supply unit (not shown) and a vacuum evacuationunit, both having a known structure. Once the environment fortransferring the wafer to the subsequent step and the environment insidethe preparatory environment compartment (vacuum condition) have becomeequal, the another valve or flange operable to introduce the wafer intothe preparatory environment compartment is opened to allow the wafer tobe introduced into the preparatory environment compartment 62, where thevacuum evacuation system or the gas supply unit as mentioned above iscontrolled to now achieve the same environment (vacuum condition) as theenvironment in which the wafer holding table 51 is located or inside thechamber 12.

After that, the gate valve 63 for isolating the chamber 12 from thepreparatory environment compartment 62 is opened to allow the wafer W asbefore the inspection to be transferred onto the wafer holding table 51(this step referred to as loading). After the transfer of the wafer asbefore the inspection having been completed, the gate valve 63 isclosed, and the environment in which the wafer holding table is locatedis adjusted to be suitable for the inspection and then the inspection isstarted.

When the wafer having finished with the inspection is carried out of thewafer holding table 51 (the operation referred to as unloading) andtransferred to the subsequent step, the operation may be carried out inan inverse order to the loading. In this regard, preferably the vacuumevacuation unit may be implemented by a combination of a turbo-molecularpump 66 with a dry-root pump 67 as shown in FIG. 1, but a rotary pumpequipped with an oil mist trap or a molecular sieve may be used insteadof the dry-root pump.

A configuration comprising a plurality (two in this case) of preparatoryenvironment compartment 62 as shown in FIG. 3 is also applicable to theillustrated embodiment.

A certain level of irradiation of the electrons onto the surface isrequired in order to obtain a capacity contrast on the surface. Toobtain this required irradiation amount (referred hereinafter to as arequired Dose amount) of electron beam, the present invention ischaracterized in having an irradiation shape of the electron beam with along beam length along a scanning direction of the stage relative to thepattern to be inspected.

The length of the beam is determined by the required Dose amount.

The required Dose amount may be controlled to achieve the best contractbetween an outer pattern and an inner pattern, and may be determined independence on the specific stage speed and the current density.

The geometry, material, sectional structure and the like of the surfaceof each specific wafer may be input, and the beam length may bedetermined from the input data and applied. In that case,Beam length X ₀ =Hz·C _(wf) ·ΔV/J _(e)

where:

Hz: Operating frequency of TDI (stage speed)

C_(wf): Electrostatic capacity of wafer surface determined in dependenceon the sectional structure, surface material and the like of the wafer,which may be determined, as follows:

by way of example, assuming that a wafer having been applied with theresist by the resist thickness of “d”, and the relative dielectricconstant of the resist is denoted by ∈_(r) and the dielectric constantin vacuum is denoted by ∈₀, thenC _(wf) =d/(∈_(r)·∈₀)ΔV=V _(EO) −V _(RTD)

V_(EO): Drawing voltage of secondary electrons

V_(RTD). Substrate voltage or retarding voltage

J_(e): Irradiated current density to substrate.

Owing to the deflection (blanking) of the electron beam, the samplesurface is subject to the irradiation of the electron beam having aslightly high density of distribution in the direction of the deflectedelectron beam. This may lead to a small bias in the charged conditionover the sample surface. To address that, a correction of the Doseamount may be applied via the blanking. To do so, an amount ofcorrection is calculated with the blanking direction aligned with thescanning direction, and a required Dose amount by “X₀” is determined.The blanking direction may be the stage scanning direction or thedirection opposite the stage scanning direction.

Assuming that the term of correction for the X₀ via the blanking isdenoted by “X_(B)”, the blanking time by “τ_(B)”, and the distance forthe beam to move on the substrate surface by “L”, thenX _(B) =Hz·L·τ _(B), and

the beam length X₀ after the correction by the blanking will be given byX ₀ =Hz·C _(wf) ·ΔV/J _(e) ±X _(B)

(It is to be noted that the sign ± depends on the blanking direction,where the + designates the blanking direction opposite the headingdirection of the stage. The sign − designates the blanking direction inconformity with the heading direction of the stage.)

Further, the correction for the required Dose amount by the blanking maybe provided by the control to the substrate voltage or the retardingvoltage for controlling the increment of the substrate surfacepotential.

In this case, the voltage adjustment ΔV_(E) will be given byΔV _(B) =J _(e)·τ_(B) /C _(wf)

In order to reduce the correction amount for the required Dose amount byblanking; the blanking direction may be set to be perpendicular to theheading direction of the stage.

Further, if there is no need for correction on the required Dose amountby blanking to be taken into account; the blanking direction may be inany desired directions.

The beam length may be changed for each specific wafer, or may bechanged depending on the sectional structure of the specific pattern tobe inspected or the specific material of the surface. For example, dataon the beam length may be previously included in a recipe, so that whenthe wafer is loaded, the beam length may be determined to be suitablefor that specific wafer or pattern to be inspected.

FIG. 22 shows a physical relationship between the irradiated electronbeam and the pattern to be inspected. In FIG. 22, the region indicatedwith the cross-hatching represents the irradiation area by the electronbeam and the size of the area. Although in the illustrated example, theirradiation area assumes a geometry with arched upper and lower edgesand straight left and right edges, the area may have a rectangular shapeor an elliptic shape that is elongated along the moving direction of thestage. As the beam starts to irradiate the pattern to be inspected andat the time when the pattern to be inspected has moved by X₀, thepattern to be inspected can pass across and under the TDI sensor. Usingthis timing, the TDI sensor starts importing of the image. In this way,the beam irradiation begins at the position by the distance X₀ away fromthe location of the TDI sensor, and the required Dose amount for theimage-taking would be given to the pattern to be inspected during thestage being moved by the distance X₀.

FIG. 23 shows another irradiation example. Depending on the arrangementof the pattern to be inspected, the X₀ may be set to be a half of thatcalculated from the required Dose amount, and in that case the stage maybe moved in a turn-back manner. In the illustrated example, the regionindicated with the cross-hatching represents the irradiation area by theelectron beam and the size of the area.

The beam irradiation begins at the position by (½)/·X₀ away from thelocation of the TDI sensor, the stage turns back after having moved by(½)/·X₀, and the stage again moves by (½)/·X₀, where the image-taking isexecuted by the TDI sensor. This allows the Dose amount equal to thatgiven by the X₀ movement to be provided to the pattern to be inspected.

In this approach, if the pattern to be inspected subsequently to thepattern to be inspected first is located downstream to the firstpattern, or located at a position opposite to the moving direction ofthe stage required for the first inspection, then the distance for thestage to travel should be cut by half, contributing to time saving.

The field of view for image-taking comprises a pixel number of the TDI,512 pix in the X direction (identical to the scanning direction of thestage in the illustrated example) and 2048 pix in the Y direction, inwhich data integration by 512 pix is configured, while the stageproviding the X-directional scanning.

The beam is configured to be elongated toward the upstream side alongthe X direction of the field of view, and the region where the TDIimports the image has been already given the required Dose amount bythis elongated electron beam thereby allowing the TDI to obtain theimage of the pattern to be inspected.

In this stage, the Y-directional dimension (beam width) of theirradiated beam may be any dimension (width) as desired depending on thedimension of the pattern to be inspected. If the larger pattern isinspected by using a low magnification factor, the beam width may besized larger than the Y-directional pixel size (e.g., 2048 pix) of theTDI, while on the other hand, if the smaller pattern is inspected byusing a high magnification factor, the beam width may be sized smallerthan the Y-directional pixel size (e.g., 2048 pix) of the TDI.

Determining the beam diameter as described above can provide a moreefficient image-taking of the overlay marks and patterns, especially foran arrangement of the overlay marks or patterns that are aligned side byside in a certain direction. Specifically, the pattern subject toimage-taking is applied with the irradiation of the primary electronsince before the timing for image-taking (this scanning referred to asPre-Dose), so that the pattern subject to image-taking should have beencharged sufficiently at the time of taking an image of the pattern. Ifthe patterns subject to image-taking are disposed serially along acertain direction, and if the beam diameter is determined in associationwith the moving of the stage as in the present invention, then each ofthe patterns subject to image-taking would be ready as they have beencharged equally for image-taking. Specifically, if the image-taking iscarried out simply by taking the distance between patterns subject toimage-taking and the stage speed into account, while moving the stage ata constant speed, an image of the pattern subject to image-taking thatwould have been held in an optimal charged condition can be takensuccessfully without the need to stop the stage each time at thelocation of the pattern subject to image-taking and stay there until thepattern subject to image-taking is charged to a constant charge amount,as is the case with the stop-and-repeat approach.

Referring now to FIG. 24, there is shown an example for measuring adistribution of lens aberration of an exposure unit with an inspectionapparatus using an electron beam.

FIG. 24 shows an example of a pattern distribution for lens aberrationresearch.

A set of isolated pattern is used as the pattern for research, and arequired number of patterns for lens aberration research is lined up ata space as required therebetween. The set of patterns is transferredonto a sample surface by the exposure unit, and any offset of that setof patterns is examined to study the distribution of the lensaberration.

In the illustrated example, the electron beam was irradiated onto thepattern over a large area, and the image is taken serially with the TDIor the EB-TDI, while providing a scanning operation by the stage or thebeam.

Since the overlay mark in the layers of a product is not applied toevery one of the chips, therefore the image is taken by repeating theprocess of step-and-repeat, while moving to the pattern section requiredto take a stationary image by the CCD or the EB-CCD using the electronbeam having a larger area to irradiate the pattern. In that case, theimage may be taken in the Still-mode of the TDI or the EB-TDI. Further,only the pattern section may be scanned for taking an image.

The invention claimed is:
 1. A method for inspecting a sample surface byusing sample surface inspection apparatus, in which an electron beamgenerated by an electron gun of the sample surface inspection apparatusis irradiated onto the sample surface, and secondary electrons emanatingfrom the sample surface are formed into an image toward an electrondetection plane of a detector for inspecting the sample surface, themethod characterized in that a condition for forming the secondaryelectrons into an image on the detection plane of the detector iscontrolled such that a potential in the sample surface varies independence on an amount of the electron beam irradiated onto the samplesurface, a method for controlling the condition for forming thesecondary electrons into an image is provided by modifying a samplevoltage or a retarding voltage in dependence on an amount of theelectron beam irradiated onto the sample surface, and the variation ofthe potential in the sample surface is computed in consideration of ablanking for preventing the electron beam from being irradiated onto thesample surface.
 2. The method for inspecting a sample surface inaccordance with claim 1, characterized in that the detector is anEB-CCD.
 3. The method for inspecting a sample surface in accordance withclaim 1, characterized in that the detector is a TDI-CCD, and adirection for deflecting the electron beam is either in the headingdirection of a stage holding the sample, or the opposite direction tothe heading direction, wherein a beam irradiation begins at a positionby a distance X₀ away from the location of the TDI-CCD sensor, and arequired Dose amount for an image-taking is given to a pattern to beinspected during the stage being moved by the distance X₀.
 4. The methodfor inspecting a sample surface in accordance with claim 1,characterized in that the detector is a TDI-CCD, and a direction fordeflecting the electron beam is either in the heading direction of astage holding the sample, or the opposite direction to the headingdirection, wherein a beam irradiation begins at the position by (½)/X₀away from the location of the TDI-CCD sensor, the stage turns back afterhaving moved by ( 1/2 )/X₀, and the stage again moves by (½)/X₀, wherean image-taking is executed by the TDI-CCD sensor.
 5. The method forinspecting a sample surface in accordance with claim 1, characterized inthat a Dose amount representing an amount of irradiated charges as per aunit area over the sample surface is calculated from the blanking and acurrent density on the sample surface.
 6. The method for inspecting asample surface in accordance with claim 1, characterized in that thesample voltage is determined by summing up an originally applied samplevoltage and a voltage applied through the irradiation of the electronbeam to the sample.
 7. A sample surface inspection apparatus,comprising: an electron gun for generating an electron beam to beirradiated onto a sample surface; a primary optical system for guidingthe electron beam onto the sample surface; a detector for detectingsecondary electrons emanating from the sample surface; and a secondaryoptical system for guiding the secondary electrons onto the detector,the apparatus characterized in further comprising a voltage adjustmentmechanism for modifying a potential in the sample surface in dependenceon an amount of the electron beam, wherein the voltage adjustmentmechanism has a means for modifying a sample voltage or a retardingvoltage in dependence on an amount of the electron beam irradiated ontothe sample surface, and the variation of the potential in the samplesurface is computed in consideration of a blanking for preventing theelectron beam from being irradiated onto the sample surface.
 8. Thesurface inspection apparatus in accordance with claim 7, characterizedin that the detector is an EB-CCD.
 9. The sample surface inspectionapparatus in accordance with claim 7 characterized in that the detectoris a TDI-CCD, and a direction for deflecting the electron beam is eitherin the heading direction of a stage holding the sample, or the oppositedirection to the heading direction, wherein a beam irradiation begins ata position by a distance X₀ away from the location of the TDI-CCDsensor, and a required Dose amount for an image-taking is given to apattern to be inspected during the stage being moved by the distance X₀.10. The sample surface inspection apparatus in accordance with claim 7characterized in that the detector is a TDI-CCD, and a direction fordeflecting the electron beam is either in the heading direction of astage holding the sample, or the opposite direction to the headingdirection, wherein a beam irradiation begins at the position by (½)/X₀away from the location of the TDI-CCD sensor, the stage turns back afterhaving moved by (½)/X₀, and the stage again moves by (½)/X₀, where animage-taking is executed by the TDI-CCD sensor.
 11. The sample surfaceinspection apparatus in accordance with claim 7, characterized in that aDose amount representing an amount of irradiated charges as per a unitarea over the sample surface is calculated from the blanking and acurrent density on the sample surface.
 12. The sample surface inspectionapparatus in accordance with claim 7, characterized in that the samplevoltage is determined by summing up an originally applied sample voltageand a voltage applied through the irradiation of the electron beam tothe sample surface.